1. Field of the Invention
The present invention relates to an assembly and process for drilling multiple subterranean wells from a cased wellbore extending to the surface, and more particularly, to such assembly and process for drilling and completing multiple subterranean wells through a template having at least two bores therethrough and being secured to casing wherein fluid is circulated to the surface during drilling via the casing.
2. Description of Related Art
Increasingly, well bores are being drilled into subterranean formations at an orientation which is purposely deviated from true vertical by means of conventional whipstock technology or a mud motor secured in the drill string adjacent the drill bit. In fractured subterranean formations, deviated wells are utilized to increase the area of drainage defined by the well within the subterranean formation, and thus, increase production of hydrocarbons from the subterranean formation. An inherent problem in utilizing a conventional whipstock to drill a deviated well is that both the depth and radial orientation of the whipstock is set when the whipstock is positioned in the well bore and cannot be changed without retrieving the whipstock from the well bore and changing the depth and/or radial orientation thereof.
In addition, wells drilled from offshore drilling platforms are usually deviated to increase the number of wells which can be drilled and completed from a single platform. Offshore drilling platforms which are utilized in deep water to drill and complete wells in a subterranean formation vary in size, structure, and cost depending upon the water depth and the loads in which the platform will be set. For example, a platform may be constructed to be supported in part by one leg or caisson which extends to the ocean floor or by as many as eight such legs or caissons. Costs of such offshore drilling platforms vary from approximately $5,000,000 US to $500,000,000 US. Each offshore drilling platform is equipped with a set number of slots via which deviated wells can be drilled and completed through casings which are secured to the platform by conventional techniques.
Due to the significant capital expenditure required for these offshore platforms, templates and processes for drilling and completing multiple cased wells have been developed. During drilling operations utilizing such templates, a conventional tubular riser is lowered into the surface or intermediate casing and inserted into one of the bores formed through the template. Once the riser is properly positioned within the bore, the surface or intermediate casing is cemented within the well bore by conventional techniques and a conventional drill string including a drill bit and mud motor (not illustrated) is transported within the riser into the bore of the template whereupon the float valve or plug and any cement is drilled out of the template bore. Thereafter, a well bore is drilled by the drill string in a conventional manner with drilling mud and formation cuttings being circulated out of the well bore to the surface via the riser. The drill string is then withdrawn from the riser and, after the well is equipped with any tubulars, the riser is withdrawn from template bore, rotated, and inserted into another bore through the template. An additional well can then be drilled and completed in a manner as just described. However, the manipulation of the riser at the surface to insert the same in and out of a given bore through a multi well template which is positioned within a cased well bore at depths of up to ten thousand feet or more and to rotate the riser for insertion into another bore may be problematic. Thus, a need exists for an assembly and process for drilling and completing multiple cased wells through a multiple well template positioned within a cased well at a subterranean location which eliminates the need to use a downhole riser to connect a subsurface or downhole template to the surface.
Accordingly, it is an object of the present invention to provide an assembly and process for drilling and completing multiple wells within subterranean formation(s) from a cased well bore which eliminates the need to utilize a riser in conjunction therewith.